The present invention relates to forming a set of cells for time difference measurements and, more specifically, to measuring such time differences for locating a user of a mobile terminal.
If someone is in trouble or notices something alarming happening and dials an emergency number, such as 112 in Europe or 911 in the U.S., the emergency services (police, ambulance, fire department) need to know where the help is needed. A caller may, especially under difficult circumstances, such as at night or in a location not known to the caller, find it extremely difficult to give route guidance for the emergency services.
Mobile terminals are widely used. Their penetration has in many countries reached and exceeded 900 of the total population. As a consequence, most people are carrying a mobile terminal with themselves while being away from home.
Cellular networks can be arranged to measure the coordinates of a mobile terminal of a subscriber under network coverage. One of the straightforward solutions then is to use this location information of the mobile terminal for locationing the subscriber who is calling an emergency number.
In the U.S., each cellular carrier is required to implement the E911 service as required by the Federal Communications Commission (FCC) in its order FCC 02-283 which, at the time of writing (Feb. 3, 2003), can be found on the Internet at the address:
http://hraunfoss.fcc.gov/edocs public/attachmatchlFCC-02-283A1.pd. Originally, the locationing accuracy in the so-called Phase II was first specified to 100 meters for 670 of calls and 300 meters for 95% of calls. From early October 2003, however, the locationing accuracy will be enhanced to 50 meters for 67% of calls and 150 meters for 95% of calls, which can be found at: http://www.fcc.gov/Bureaus/wireless/News Releases/2001/nw 1012 7a.pdf.
For the Global System for Mobile communications (GSM), the locationing is usually performed by using the so-called Enhanced Observed Time Difference E-OTD method.
It is possible to make the locationing measurements in idle mode, this kind of approach being used by some manufacturers. Typically, the locationing interval is approximately 10 seconds. Then, the locationing history is already known by the mobile terminal MS when it enters dedicated mode. Not only the location but also speed and direction, if any, can be determined.
Other manufacturers do not use idle mode measurements. Rather, the location of the mobile terminal is found out only at the beginning of each 911 or 112 call. Presently, such manufacturers have at least some problems in obtaining the desired accuracy. According to some opinions, in order to achieve the accuracy requirements it is necessary to execute idle mode measurements for neighboring cells.
The problem with the state-of-the-art solutions is that the continuously performed measurement of time differences consumes a great deal of power, significantly reducing the standby time of the mobile terminal. As such, the battery of the mobile terminals needs to be recharged quite often. This is an undue burden for most mobile subscribers because the mobility of the user is, obviously, substantially limited while the battery of the mobile terminal is being recharged.
One solution for channel measurements, as disclosed in international patent application WO 2001/58201, is for the mobile terminal to receive a neighbor cell list from a base station. Channel quality measurements are performed for the cells on the list based on the location of the mobile device. This is performed in order to reduce the power consumption in the terminal.
The solution proposed in WO 2001/582-1 is, however, far from optimal for locationing measurements. It cannot be used if the location of the mobile station is not known. Further, it has turned out to be extremely difficult to construct neighbor cell lists in such a manner that they would not only provide enough cell reselection possibilities but also enable good enough quality for time difference measurements still saving power in the mobile terminal.
EP 0 930 513 A2 presents a cellular radio network based positioning system for determining the position of a mobile station. For each base transceiver station or cell of the network, a fixed list of base transceiver stations is stored by a mobile positioning center. Each list identifies those base transceiver stations which enable the position of a mobile station served by the corresponding base transceiver station to be optimally determined. The list is transmitted to the mobile station via the serving base transceiver station and the mobile station determines an observed time difference for each of the listed base transceiver stations, relative to the serving base transceiver station, from signals broadcast by the listed base transceiver stations. The observed time differences are transmitted from the mobile station to the serving base transceiver station and are used by the network to compute the position of the mobile station.